1. Field of the Invention
This invention relates to the deposition of metals as interlayers within films.
2. Background of the Invention
Organic polymeric films are often combined with metals (metallized) for various kinds of practical applications requiring such properties as electrical conductivity, optical density or reflectivity. A distinction may be drawn between methods which involve blending metal particles or metal precursors (such as metal salts, see for example, A. K. St. Clair and L. T. Taylor, J. Appl. Poly. Sci., Vol. 28, 2393-2400 (1983); and A. Auerbach, J. Electrochem. Soc., 937, (1984)) with a polymer melt, solution, or latex prior to fabrication of the film, and those methods which combine the metal with a preexisting film. Among the latter methods, we may further distinguish between methods which metallize only a surface of the film (for example, lamination, vapor deposition, or electroless deposition, see R. W. Berry, P. M. Hall, and M. T. Harris, "Thin Film Technology", D. van Nostrand Co., Inc., Princeton, N.J., 1968, pp 1-17) and methods which introduce metals within a preexisting solid polymeric film. By "within" we mean distributed in some volume element between the two surfaces of the film. Said volume element may or may not extend to the external surfaces of the film.
Existing art includes two ways in which metals may be introduced within preexisting solid polymeric films. The most common example is photographic technology (see, for example, A. Roff and E. Weyde, "Photographic Silver Halide Diffusion Processes", The Focal Press, London and New York, 1972, pp 13-31). Conventional photographic methods produce metal particles dispersed within a polymer film wherein their primary purpose is to absorb light. These particles are generally dispersed to such a degree that they do not constitute a continuous phase and therefore lack the macroscopic properties of electrical conductivity and optical reflectivity commonly associated with surface-metallized structures. U.S. Pat. No. 4,512,855 discloses a metal interlayer deposition (MID) process. This process involves reduction of metal ions, diffusing from one surface of a polymer film, by electrons supplied via redox reactions of the polymer, the electrons being supplied from the opposite surface of the film. In this process it is necessary that the polymer be electrochemically active in the sense that it is capable of undergoing reversible redox reactions at appropriate potentials to reduce the metal to its zero-valent state. Among the unique and desirable features of the MID process is the fact that it is capable of producing metal interlayers of sufficient density that they are electrically conductive, and with sufficiently smooth and sharply defined surfaces that they are optically reflective. Typically, under normal operating conditions, only the side of the interlayer facing the solution is sufficiently sharply defined to be reflective, the other surface being obscured by diffusely distributed metal particles.
Precipitation reactions, including some of the simplest sorts of chemical processes, can give rise to remarkably varied and complex morphologies, particularly when they are kinetically coupled to physical transport. For example, consider the reaction between two species A and B to yield an insoluble product C under conditions where either one or both of the reactants is/are supplied by diffusion across a convection-free reaction medium, such as a gel. Many such systems have been studied. The range of morphological complexity observed for C includes examples of periodic multilayers (Liesegang rings), K. H. Stern, Chem. Rev., 54, 79 (1954); large single crystals, H. K. Henisch, "Crystal Growth in Gels", Pennsylvania State University Press, 1970; and dense, sharply defined interlayers (so-called "precipitate membranes"), A. J. Ayalon, Membrane Sci., 20, 93 (1984).
A particular experimental arrangement, which we refer to herein as counter-current diffusion, involves diffusing A and B together from opposite ends of a gel column or the two sides of a film. This situation has been studied by a number of researchers, including A. E. Nielson, A. Hunding and B. Pokric, Croatica Chemica Acta, 50, 39 (1977); D. Srzic, B. Pokric and Z. Pucar, Zeit. Phys. Chem. N. F., 103, 157 (1976); E. P. Honig , J. H. Hengst and P. Hirsch-Ayalon, Ber. Bunsenges. physik. Chem. 72, 1231 (1968); A. C. Allison and J. H. Humphrey, Nature, 183, 1590 (1959); C. J. VanOss, J Colloid and Interface Sci., 27, 684 (1968); and Z. Pucar, B. Pokric and A. Graovac, Anal. Chem., 48, 403 (1974), and certain important generalizations have been established. For example, in cases where a single dense interlayer is deposited, such as BaSO.sub.4 precipitation in agar, Honig, Hengst and Hirsch-Aylon, supra, the position of the interlayer is purely kinetically controlled, that is, the position corresponds to the point at which the concentrations of Ba.sup.+2 and and SO.sub.4.sup.-2 are equal to one another and equal to some minimum critical value. Moreover, this concentration is specifically not given by the equilibrium solubility products rule: K.sub.sp =[C(A)][C(B)]. In fact, the critical concentration may exceed the equilibrium solubility by many orders of magnitude, Srzic, Pokric and Pucar, supra.
Studies of counter-current precipitation in polymer films have been less extensive than in gels. Although the formation of a BaSO.sub.4 interlayer in cellulose film has been reported long ago, P. Hirsch-Ayalon, Rec. Trav. Chim., 75, 1065 (1956), the morphology of that interlayer was not characterized. One such example in cellophane was reported to be only a few .mu.m thick; however, neither the thickness of the film nor the position of the interlayer was given, Honig, Hengst and Hirsch-Ayalon, supra. A report, K. F. Mueller, Science, 225, 1021 (1984), concerning precipitation of insoluble salts in poly(vinyl alcohol) films includes an example of complex multilayer formation (similar to Liesegang rings). It appears that, to a certain extent, the dimensions of precipitated interlayers scale with the thickness of the reaction medium.
It is an object of this invention to provide a process by means of which zero-valent metal can be deposited within a film. Another object is to provide a process by means of which, if desired, the metal can be deposited as an interlayer within the boundaries of the film surfaces. Another object is to provide a process by means of which the position of the interlayer in the film can be systematically controlled so as to be any finite distance from the surface of the film. Still another object is to provide a process by which multiple interlayers can be deposited within a single film in a systematically controlled manner. A further object is to provide control over the width of the resultant interlayer by changing the chemical reaction rate between the metal ion and the reducing agent. Still another object is to provide a process which, if desired, will yield a metal interlayer having sufficient density, continuity and surface regularity to exhibit electrical conductivity and optical reflectivity which are characteristic of bulk metallic phases or surface coatings. A further object of the invention is to provide a process that does not require an electroactive polymer, that can be carried out in a single step on a free standing film of variable geometry, and that can be used to produce interlayers having sharp boundaries so as to make the film more reflective on both sides. These and other objects will become apparent hereinafter.